Re: [PATCH 7/7] ixgbevf: eliminate duplicate barriers on weakly-ordered archs

On 3/14/2018 9:44 PM, Alexander Duyck wrote:

On Wed, Mar 14, 2018 at 3:57 PM, Sinan Kaya [off-list ref] wrote:

quoted

Hi Alexander,

On 3/14/2018 5:49 PM, Alexander Duyck wrote:

quoted

On Wed, Mar 14, 2018 at 5:13 AM,  [off-list ref] wrote:

quoted

On 2018-03-14 01:08, Timur Tabi wrote:

quoted

On 3/13/18 10:20 PM, Sinan Kaya wrote:

quoted

Actually I would argue this whole patch set is pointless. For starters
why is it we are only updating the Intel Ethernet drivers here?

I did a regex search for wmb() followed by writel() in each drivers directory.
I scrubbed the ones I care about and posted this series. Note also that
I have one Infiniband patch in the series.

I didn't see it as it I was only looking at the patches that had ended
up in intel-wired-lan. Also was there a cover page, I couldn't seem to
find that on LKML.

Yeah, I didn't have a cover page. These patches were sitting on my branch
for a while. I wanted to get them out without putting too much effort into
it. I'll add it on the next version.

quoted

I considered "ease of change", "popular usage" and "performance critical
path" as the determining criteria for my filtering.

It might be advisable to break things up to subsystem or family. So
for example if you are going to update the Intel Ethernet drivers I
would focus on that and maybe spin the infiniband patch of into a
separate set that can be applied to a separate tree. This is something
I would consider more of a driver optimization than a fix. In our case
it makes it easier for us to maintain the patches to the Intel drivers
if you could submit just those to Jeff and Intel-wired-lan so that we
can take care of test and review as well as figure out what other
drivers will would still need to update in order to handle all the
cases involved in this.

quoted

quoted

This
seems like something that is going to impact the whole kernel tree
since many of us have been writing drivers for some time assuming x86
style behavior.

That's true. We used relaxed API heavily on ARM for a long time but
it did not exist on other architectures. For this reason, relaxed
architectures have been paying double penalty in order to use the common
drivers.

Now that relaxed API is present on all architectures, we can go and scrub
all drivers to see what needs to change and what can remain.

My only real objection is that you are going to be having to scrub
pretty much ALL the drivers. It seems a little like trying to fix a
bad tire on your car by paving the road to match the shape of the
tire.

Or we start with mostly used ones and hope to increase the coverage over time.
It will take a while to cover all drivers.

We could do several iterations like you are suggesting for each subsystem.

quoted

quoted

It doesn't make sense to be optimizing the drivers for one subset of
architectures. The scope of the work needed to update the drivers for
this would be ridiculous. Also I don't see how this could be expected
to work on any other architecture when we pretty much need to have a
wmb() before calling the writel on x86 to deal with accesses between
coherent and non-coherent memory. It seems to me more like somebody
added what they considered to be an optimization somewhere that is a
workaround for a poorly written driver. Either that or the barrier is
serving a different purpose then the one we were using.

Is there a semantic problem with the definition of wmb() vs. writel() vs.
writel_relaxed()? I thought everything is well described in barriers
document about what to expect from these APIs.

AFAIK, writel() is equal to writel_relaxed() on x86 architecture.
It doesn't really change anything for x86 but it saves barriers on
other architectures.

Yeah. I had to go through and do some review since my concerns have
been PowerPC, IA64, and x86 historically. From what I can tell all
those architectures are setup the same way so that shouldn't be an
issue.

OK, glad that we are in common understanding.

quoted

quoted

It would make more sense to put in the effort making writel and
writel_relaxed consistent between architectures before we go through
and start modifying driver code to support different architectures.

Is there an arch problem that I'm not seeing?

Sinan

It isn't really an arch problem I have as a logistical one. It just
seems like this is really backwards in terms of how this has been
handled. For the x86 we have historically had to deal with the
barriers for this kind of stuff ourselves, now for ARM and a couple
other architectures they seem to have incorporated the barriers into
writel and are expecting everyone to move over to writel_relaxed. 

You want to move to writel_relaxed() only if you know that your register
accesses won't have any side effects. 

If you require some memory update to be observable to the HW before
doing a register write, right thing is to do 

wmb() + writel_relaxed()

wmb() + writel() is clearly the wrong choice and that's what the goal of
this change.

If we know that all writel() adaptations in all architectures guarantee
observability, another option is to get rid of wmb() and just keep writel().

I'm not so convinced about this and hoping that someone will correct me. 

wmb() on x86 seems to have an sfence but writel() seems to have a compiler
barrier in it. So, the type of barrier wmb() is using is different. 

we can't say

(wmb()+ writel_relaxed()) == writel()

for all architectures, but maybe I'm wrong.

We really don't want to convert all writel() to writel_relaxed() blindly
without giving much thought into it.

This was also another reason why I limited the changes to wmb() + writel()
combinations only.

If there is wmb() + code + writel() and if we convert this to wmb() + code +
writel_relaxed(), code will not be observed by the HW and this might break
the driver.

It
seems like instead of going that route they should have probably just
looked at pushing the ARM drivers to something like a "writel_strict"
and adopted the behavior of the other architectures for writel.

I'll go back through and review. It looks like a number of items were missed.

OK, I'll take a look at each one. Saw the code concern on the e1000e suggestion.
I wanted to raise it here first before inspecting the rest.

-- 
Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.